1. Field of the Invention
This invention relates to transmitting and receiving antennas, and, in particular, to antennas including a plurality of conductive transceiver elements having a plurality of turns.
2. Background Information
There is considerable incentive to decrease the height of antennas from that of the towering dipole to a more diminutive form while maintaining similar levels of efficiency and radiation pattern. It has long been thought that a horizontally oriented magnetic flux ring would be the best form for achieving this goal, although the implementation of a uniform magnetic flux ring is not simple or straightforward.
U.S. Pat. Nos. 4,622,558, 5,442,369, and 6,028,558 disclose three such attempts at producing rings of magnetic flux and, thereby, approaching the goal of dipole like radiation patterns. While each reference may achieve a different level of success, their weakness is that standing waves of current are not uniform about a toroidal surface and, hence, the ring of magnetic flux is not uniform. Therefore, the radiation pattern deviates from that of a dipole. See, also, U.S. Pat. Nos. 5,734,353; and 5,952,978.
U.S. Pat. No. 5,442,369 discloses, for example, an omnidirectional poloidal loop antenna employing inductive loops (FIG. 27), a cylindrical loop antenna (FIG. 31), a toroid with toroid slots for tuning and for emulation of a poloidal loop configuration (FIG. 33), and other toroidal antennas employing a toroid core tuning circuit (FIG. 34), a central capacitance tuning arrangement (FIG. 36), a poloidal winding arrangement (FIG. 37), and a variable capacitance tuning arrangement (FIG. 38).
The embodiments of FIG. 27 and 31 of U.S. Pat. No. 5,442,369 share the disadvantage of relatively large size because of the necessity for the poloidal loop circumference to be on the order of one half wavelength for resonant operation. U.S. Pat. No. 5,442,369 teaches that the loop size may be reduced by adding either series inductance or parallel reactance to those structures.
U.S. Pat. No. 5,654,723 discloses antennas having various geometric shapes, such as a sphere. For example, if a sphere is small with respect to wavelength, then the current distribution is uniform. This provides the benefit of a spherical radiation pattern, which approaches the radiation pattern of an ideal isotropic radiator or point source, in order to project energy equally in all directions. Other geometric shapes may provide similar benefits. Contrawound windings are employed to cancel electric fields and leave a magnetic loop current.
Referring to FIG. 1 hereof, two helical windings 2, 4 of a Contrawound Toroidal Helical Antenna (CTHA) 6 are shown. CTHAs are disclosed, for example, in U.S. Pat. Nos. 5,442,369; and 6,028,558, which are incorporated by reference herein. The contrawound helical windings 2, 4 are fed with opposite currents in order that the magnetic flux of each helix reinforces the loop magnetic flux. This additive effect of the two helices may produce a stronger magnetic flux than a single toroidal helix (not shown), but the magnetic flux is not uniform. The effect can approach uniform currents for an electrically small CTHA, but suffers poor efficiency.
FIG. 2 shows a plot 8 of the currents in the two helical windings 2, 4 of FIG. 1 at the half wavelength resonance as predicted by the Los Alamos National Laboratory's Numerical Electromagnetics Code (NEC). These non-uniform currents, in turn, produce non-uniform magnetic fields.
As shown in FIG. 3, the exemplary NEC simulation from FIG. 2 provides a plot 10 of a 3D-radiation (i.e., .theta. plus .phi.) pattern having two dimples (only one dimple 12 is shown). This pattern about the X-Y-Z origin 14 is considerably different from the radiation pattern of a dipole (not shown). While not all CTHA antennas have as pronounced a dimple as the dimple 12, those antennas all share the characteristic of near isotropic radiation (i.e., there is no overhead null).
Since the best gain for an isotropic radiator is, by definition, 0 dBi, and the best gain of a dipole is about +2.5 dBi (e.g., about +2.57 to about +2.74 dBi), applications that only need azimuthal (e.g., horizontal in the exemplary embodiment) patterns suffer an apparent disadvantage when employing a CTHA. For these applications, there exists the need for a uniform magnetic ring.
Although the prior art shows various antenna structures, there is room for improvement.